This invention relates generally to radiological well logging methods and apparatus for investigating subsurface earth formations traversed by a borehole and, more particularly, to an improved method and apparatus for pulsed neutron gamma ray logging for identifying and measuring elements in subsurface formations.
A number of well logging techniques in use come under the general classification of neutron induced radiological well logging. In their basic form, these techniques involve irradiating subsurface earth formations with high energy neutrons and determining the effects of formation constituents on the irradiating neutrons by measuring gamma rays produced by inelastically scattered neutrons or the gamma rays resulting from thermal neutron capture. In the case of inelastic scattering, part of the energy lost by the irradiating neutrons is released in the form of gamma rays at the instant of collision. Whereas, a capture reaction is where a thermalized neutron is absorbed into the nucleus of an atom and ceases to exist independently. In most such capture reactions, one or more high energy gamma rays are generated.
Typical well logging systems for induced gamma ray logging employ a subsurface instrument including a neutron source for pulsatingly irradiating earth formations and a crystal detector coupled to a photomultiplier tube as a detector system. In the prior art embodiments the crystal portion of the detector system has employed a NaI(Tl) crystal suitably sized to be contained within the elongated housing of the subsurface instrument. While the NaI(Tl) crystal has provided acceptable information, the use of this crystal has proven to be a serious limitation.
The NaI(Tl) crystals employed in the art have a relatively low density resulting in relatively low counting efficiency. Additionally, these crystals, due to pair production characteristics, exhibit a full-energy-to-escape-peak ratio such that the energy bands required to measure a particular formation constituent must be set to encompass a main photopeak and first and second escape peaks. Use of an energy band of such width substantially increases the background contribution to the desired measurement reducing the accuracy thereof.
Accordingly, the present invention provides improved methods and apparatus for measuring and identifying characteristics of subsurface formations from detected gamma radiation resulting from neutron inelastic scattering and thermal neutron capture.